In filtering phase or frequency modulated signals it is particularly important not to introduce phase nonlinearities into the signal since such nonlinearities cannot be effictively corrected or compensated (as distinguished from linear distortion). Thus, if the components of, for example, a composite stereo sound signal including an FM component, are to be separated without nonlinear distortion, the filters employed for separation must be phase linear at least over the pass-bands of the respective components.
Desirably, such filters will have a boxcar shaped frequency response and be phase linear and distortionless across the pass-band. Practically, such filters are difficult and expensive to realize.